Process for preparing conjugate filaments

ABSTRACT

CONJUGATE FILAMENTS OF POLYPROPYLENE ARE HEAT TREATED PRIOR TO DRAWING TO DEVELOP MAXIMUM CRIMPING POTENTIAL IN A REDUCED TIME. THE HEAT TREATMENT IS CARRIED OUT BY FOCUSING RADIENT HEATERS ON THE UNDRAWN YARN PACKAGE AS IT IS BEING WOUND. THE HEATERS ARE MAINTAINED AT A CONSTANT DISTANCE FROM THE YARN PACKAGE AND A TEMPERATURE OF 120 TO 200*C. IS MAINTAINED IN THE SPACE BETWEEN THE HEATERS AND THE YARN PACKAGE.

April 11, 1972 J. M. BUCHANAN PROCESS FOR PREPARING GONJUGATE FILAMENTS Filed July s, 1970 JACK M. BUCHANAN INVENTOR FlG .2

BY wiw. 1W

ATTORNEY United States Patent 3,655,859 PROCESS FOR PREPARING CONJUGATE FILAMENTS Jack M. Buchanan, Parkwood, N.C., assignor to Hercules Incorporated, Wilmington, Del. Filed July 8, 1970, Ser. No. 53,048 Int. Cl. B29c 25/00; D01d 5/22 US. Cl. 264-234 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to the preparation of high bulk yarns based on composite filaments of polypropylene. In particular, it relates to a process for heat treating such a yarn prior to the drawing thereof.

In copending US. patent application Ser. No. 587,780, there is taught a process for preparing conjugate polypropylene filaments comprised of at least two components of different molecular weight distributions. When yarns of these filaments are drawn and relaxed under proper con ditions, the different components exhibit differing propensities for shrinkage, with the result that the filaments and the yarn are distorted into a helically crimped, high bulk configuration.

In developing the aforesaid method, it was determined that, in order to develop the maximum or optimum crimp potential in the filament, it is necessary that the undrawn yarns be aged prior to being drawn. At room temperature, aging and development of optimum crimp potential take place at an exponentially decreasing rate over about 7 to 14 days. It is postulated that the aging period permits the polymer to crystallize to some optimum structure for subsequent crimp development. It may also be that the aging permits the relaxation of strains set up in the filaments during spinning and draw-down. Obviously, the time delay occasioned by this waiting period is an economic drawback to the conjugate filament process.

It has also been determined that the rate of aging of the conjugate yarn can be significantly increased by reheating the yarn to about 80 to 140 C. for a short time. An effective and practical method of applying this heat uniformly has not heretofore been devised, however. The best method devised to date has been the use of heated godet rolls. These are effective with single yarn ends but due to space limitations, have not been parctical when using multiple-end take-up units. Heating in an oven is impractical because the poor heat transfer of the yarn results in non-uniform treatment of the yarn throughout the package.

It is the purpose of this invention to provide a method and apparatus for applying heat to effect the development of maximum or optimum crimp potential at an accelerated rate using an equipment arrangement which is easily installed on conventional yarn collection apparatus. The improved method of the invention comprises subjecting the undrawn yarn, while in the process of being wound on a bobbin or other type of package, to a radiantly heated environment of about 120 to 200 C.

The significant feature of this invention by which the accelerated aging of the yarn is effected is the heat treatment of the yarn while it is being wound onto a bobbin or other collection package. The yarn is rapidly heated by means of high energy radiant heat provided by a heater focused on the package during the winding. Thus any given portion of the yarn is first exposed to the maximum output of the heater for a relatively short time, following which it is exposed to gradually diminishing heat as successive layers of yarn are laid on it, shielding it from the heat source and moving it farther from the heat source. Additionally, the yarn as it is covered over by successive layers, cannot give up heat to these successive layers since the successive layers are warmer. Thus the yarn in the center of the package is not able to cool to room temperature for a relatively long period of time, i.e., at least not until the package has been dofied from the winder. Even after removal from the winder the package cools only slowly due to the poor thermal conductivity of the polypropylene. The net result of the process is to subject the yarn to quite a long exposure to elevated temperature without any additional handling steps.

The invention and equipment for carrying it out are illustrated in the attached drawing, in which:

'FIGS. 1 and 2 are front and rear views of yarn winding apparatus havinga radiant heater installed thereon.

The apparatus depicted in FIGS. 1 and 2 comprises a yarn winding frame 1 including a yarn package supporting member 3, and a plurality of yarn packages 5, and having radiant heaters 7 mounted thereon, supported by support members 9. Associated with the radiant heaters are a pneumatic sensing element 11, air control valve 13, pneumatic cylinder 15 and heater retracting arm 17.

In carrying out the process of the invention, the yarn end 2 from the spinning head (not shown) are collected on bobbins or other collection devices to form packages 5. As the packages are wound, they are subjected to a heat environment from radiant heaters 7 focused on the surface of the package.

As the yarn package increases in diameter, the heaters are retracted at the same rate of increase of package diameter to maintain a constant spacing between the heaters and the yarn package. Retraction of the heaters is effected by the cooperation of the sensing element 11 and pneumatic cylinder 15. The heaters are held in place with respect to the yarn package by pneumatic cylinder 15 adapted to actuate heater retracting arm 17 as the air pressure thereon changes. Air is supplied to pneumatic cylinder 15 through air control valve 13. A side stream of air from air valve 13 pressurizes lines 19 and 21 leading, respectively, to and from sensing element 11. A jet of air is emitted by sensing element 11 into contact with one of the yarn packages. As the yarn package diameter increases, the back pressure in line 21 increases, actuating air control valve 13 to increase the air flow to pneumatic cylinder 15 which in turn retracts heater 7, via retracting arm 17 until the equilibrium pressure is regained.

The heat required to be applied to the yarn during the windup heat treatment is the amount of heat that will impart to the yarn a process relaxation of at least 55% when it is drawn and relaxed immediately upon being doffed from the windup station. By process relaxation is meant the sum of the loss in length of a filament caused by linear shrinkage plus that caused by crimp formation.

It is found that a radiant electric heating source opcrates at a sufficiently constant output to maintain a constant emperature in the area between the heater and the yarn. Since the temperature of the yarn per se is not readily determined, the heat input to the yarn is specified in terms of the temperature of the environment through which it passes in being collected under the influence of the heat. This temperature should be at least about C., preferably at least about 140 C., and preferably not above about 200 C. The primary control of the heat applied to the yarn is effected by varying the spacing between the yarn package and the radiant heat source. The determination of the precise heater spacing relative to heater capacity and yarn advancement rate is within the skill of the art.

As shown in the drawing, the spacing between the heater and the yarn package is sensed and maintained pneumatically. Other means known to the art can also be employed. For example, a photoelectric or other type of electronic sensing device could be employed. It is preferred that the sensing element be one which does not make physical contact with the yarn package because such contact can distort the package, can catch on the yarn or can cause other types of damage.

Except for the use of heat during winding of the undrawn yarn, the process employed is the conventional conjugate filament process described in the above mentioned U.S. Ser. No. 587,780. Briefly, the two (usually two, although a greater number can also be used) components are coextruded through a specially designed spinneret wherein the components contact each other in the molten state as they leave the spinning orifices. Spinning is normally carried out at a temperature within the range of about 240 to 320 C. The spun filaments udergo a nonorienting draw-down in the melt and are quenched as rapidly as possible upon leaving the spinneret. A plurality of these filaments are gathered into a yarn and this yarn is the product with which the heat-treatment step of this invention is employed.

The starting materials contemplated for use herein are highly crystalline polymers made Wholly or predominantly from propylene and having differing molecular weight distributions and intrinsic viscosities of at least 1.9 which can be the same or different for each polymer. These polymers include crystalline polypropylene itself, and crystalline copolymers of propylene with up to about mole percent of ethylene. The molecular weight distribution of the polymers is expressed in terms of a dispersion coefficient which is the ratio of weight average molecular weight, Mw, to number average molecular weight, Mn. The values of Mw and Mn can be determined as described in an article by Shirley Shyluk, J. Poly. Sci. 62, 317 (1962), entitled Elution Fractionation of Atactic and Isotactic Polypropylene.

The dispersion coefiicient of the polymer having the higher value should be at least 4 and that of the polymer having the lower value should be at least about 2.5. The difference between the dispersion coefiicients of the polymer components should be at least 1.0 in order to obtain the highly crimped filaments of the invention. Polymers having dispersion coeificients up to about 20 can be used.

Polymers having different molecular weight distributions can be produced in several ways. Thus, thermal degradation of polypropylene from a higher to a lower viscosity will cause a narrowing of the molecular weight distribution, the product then being used as one component of a composite filament, the other component being, for example, an undegraded polymer or a polymer which has been subjected to a lower degree of degradation. Alternatively, a polymer, a portion of which has been chain terminated during polyerization as, for example, by the use of hydrogen as chain terminator, and which has a broad molecular weight distribution, can be used as the second component. The ratio of the component having the broader molecular weight distribution to the component having the narrower molecular weight distribution can be varied from about 1:1 to about 3:1, the selfcrimping eifect obtained over this range being somewhat dependent upon the difference between the dispersion coefiicient for each component. Thus, as the ratio is increased, the difference in dispersion coefficient should also be increased.

In selecting suitable propylene polymer components for use in the preparation of filaments and fibers according to the invention, it is necesary to use polymers which, in addition to having a diiference between dispersion coefficients of a selected pair of at least about 1.0, also have intrinsic viscosities of at least about 1.9, since the high level of crimp herein contemplated is not obtained wtih polymers of lower intrinsic viscosity, particularly in the case of the higher denier filaments and fibers, and/or if the difference in dispersion coefficients for the pair is low. It is preferred to use polymers having intrinsic viscosities from about 2.0 to about 2.5. Polymers having intrinsic viscosities above about 3.2 should not be used because of the tendency toward melt fracture.

A satisfactory crimp level can be obtained by using polymer components having the same or different intrinsic viscosities provided their dispersion coefficients differ by at least 1.0. Thus, polymer components having intrinsic viscosities which differ by, for example, 0.9 have given satisfactory results.

The component polymers may contain up to about 20% or more by weight of other substances as, for example, stabilizing substances, delustrants or pigments or substances conferring an afiinity for dyestuffs on the propylene polymers.

Drawing can be carried out on conventional equipment using feed and draw rolls, the latter rotating at a sufficiently higher rate of speed than the former to accomplish the desired draw. In order to come within the critical temperature limits for drawing, the feed roll temperature should not exceed about 70 C. and the draw roll temperature should not exceed about C. and can be as low as room temperature. Drawing is usually carried out to a level of 200% to 600% of the original length.

From the draw roll the filament can be passed to a relaxation roll which should be maintained at temperatures from about room temperature to about 140 C. for optimum crimp formation and permanency. Alternatively, inprocess relaxation can be carried out by conventional and modified means such as a hot air chamber, hot plate, radiation heating devices inserted between the draw and relaxation rolls, etc., to accomplish spontaneous crimp development and effective heat setting. The crimp level can be controlled conveniently by adjusting the speed of the relaxation roll in a range from about 100% to about 50% of the draw roll speed and the temperature of the filament between the draw and relaxation rolls sufliciently high, but below the melting point of the filament to obtain the desired relaxation. Relaxation can also be carried out on skeins of yarn in a hot air oven at about C. for about 30 minutes. Since relaxation is a function of time and temperature, crimp will develop at low temperatures, e.g., room temperature, if the time is great enough.

If desired, the relaxation step can be omitted at this point and the drawn yarn wound on a bobbin under tension to produce yarn having a latent crimp. This yarn can then be formed into fabric as by knitting or weaving and the fabric then given a treatment, e.g., a scour and heat treatment to develop the crimp.

In the following examples, which illustrate the invention, two parameters are used to characterize the drawn, relaxed and crimped yarns. The process relaxation refers to the amount, expressed as a percentage, by which the length of the drawn yarn decreases during the relaxation step, including both linear shrinkage and loss due to the formation of helical crimps. The crimp energy is the work, expressed in inch pounds/denier, required to elongate a length of yarn until essentially all crimp is removed. This parameter is an indication of the crimps per inch in the yarn, i.e., higher values of crimp energy indicate greater crimps per inch level, but is more easily and more precisely determined than crimps per inch.

It. will be noted that, in some instances, yarns treated according to this invention and others which are untreated, when relaxed to the same extent exhibit difierent crimp energy values. This is an indication that the frequencyamplitude relationship of the crimps in the two products is different due to differences in the linear shrinkage of the components. When both are relaxed to the maximum and both have developed their maximum crimp, the crimp energy is essentially the same.

EXAMPLE 1 A series of bicomponent yarns were prepared composed of equal amounts of an undegraded polypropylene having an intrinsic viscosity of 2.05 and a thermally degraded component having in intrinsic viscosity of 2.0. The differential between the dispersion coefiicients of the components was about 4.

The two components were coextruded through a conjugate spinning head at about 265 C., subjected to a non-orienting drawdown of about 166 and collected into a 260 denier, 36 filament yarn at a rate of about 500 yards/ minute. This yarn was wound on a bobbin and during the winding, was passed under two 1000 watt radiant heaters each having a face of 2 x 18 inches in area focused on the yarn package and adapted to maintain a /z inch gap from the package. The temperature in the space between the heater and the yarn package was about 170 C.

A control series was also prepared following the same procedure except that the heat treatment step was omitted.

Within 15 minutes of dofiing, specimens of both yarns were cold drawn 300% at 55 C. between differentially driven rolls and, in a continuous operation, they were relaxed at 140 C. under substantially no tension using a modified draw twister.

Crimp characteristics of the resulting products were as follows:

Process relaxation Crimp energy Control 35 percent* 4.35 lnchlb./denier. Example 1 55 percent"... 12.49 inch lb./denier.

*Maximum attainable.

A second specimen of the untreated control was aged for 7 days, then drawn as above, relaxed at 140 C. under no tension so that its process relaxation was the same as that of Example 1, namely 55%. However, the crimp energy of-,this yarn was only 8.03 inch lbs./denier.

EXAMPLE 2 Process relaxation Crimp energy (a) Heat treated 65 percent 14.62lnch lb./denler. (b) Control 65 percent 11.75 inch lb./denier. Heat treated 70 pereent. 16.43 inch lb./denler. ((1) Control 70 pereent 13.97 inch lb./denler.

As the crimp energy data indicate, the heat treated yarns possess a different crimp frequency-amplitude relationship and a greater level of crimps per inch than do those which have aged naturally, indicating that 7 days aging at room temperature did not develop as great a crimping potential in the yarn as was developed by the method of the invention.

EXAMPLE 3 Crimp energy Room temperature 135 C.

Age Treated Control Treated Control 15 minutes 1e. 63 3.58 23. 25 4. 10 4 hours- 17. 15 9. 61 23. 84 18. 86 2 days. 18. 86 12. 82 24. 28 22. 62 4 days. 16. 06 12. 67 24. 14 23. 06 8 days- 15. 42 12. 39 23 30 22. 71

It will be noted that the heat treated yarn had reached its maximum crimp potential substantially immediately upon its formation whereas the control yarns required substantially more time to reach maximum potential. It will also be noted that in this case, where the treatment was more severe than in the continuous process employed in the previous examples, the ultimate level of crimp obtained was substantially the same.

What I claim and desire to protect by Letters Patent is:

1. In the process of preparing a yarn wherein at least two propylene polymer components having diflerent molecular weight distributions are conjugately spun to form composite filaments and said composite filaments are gathered into an undrawn yarn, wound into packages, and thereafter unwound and drawn about 200 to 400% and heat relaxed to elfect helical crimping, the improvement which comprises subjecting the undrawn yarn, while in the process of being collected into packages, to a radiantly heated environment of about to 200 C., said heat being applied to the yarn at a point beyond the point where the yarn first contacts the package.

References Cited UNITED STATES PATENTS 2,289,860 7/1942 Babcock 264-210 F 3,215,486 11/1965 Hada et a1 2'64-210 F 3,323,190 =6/1967 Boltniew 264-210 F 3,323,165 6/1967 Mottern et al. 264-167 3,343,207 9/1967 Mottern et al. 264-167 3,382,306 9/ 1968 Oppenlander 264-167 3,399,108 8/1968 Olson 264-171 3,399,259 8/ 1968 Bragford 264-171 3,491,178 1/1970 Nishioka et a1 264-171 3,505,164 4/1970 Oppenlander 264--171 JAY H. WOO, Primary Examiner US. Cl. X.R. 

